Preparation and Thermally Promoted Ripening of Water-Soluble Gold

Aug 23, 2007 - Theory Comput. J. Comb. Chem. J. Med. ... Langmuir , 2007, 23 (20), pp 10262–10271 ... View: PDF | PDF w/ Links | Full Text HTML ... ...
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Langmuir 2007, 23, 10262-10271

Preparation and Thermally Promoted Ripening of Water-Soluble Gold Nanoparticles Stabilized by Weakly Physisorbed Ligands Fionn Griffin* and Donald Fitzmaurice Department of Chemistry, UniVersity College Dublin, Belfield, Dublin 4, Ireland ReceiVed May 5, 2006. In Final Form: June 1, 2007 Here, we report on a facile method for the preparation of an aqueous dispersion of 3.6 nm gold nanoparticles electrostatically stabilized by a weakly physisorbed ligand, namely, 4-(dimethylamino)pyridine (DMAP). The nature and extent of the interaction of this ligand with the surface of a gold nanoparticle has been examined. We also report on the thermally promoted ripening of these nanoparticles under mild conditions to yield a dispersion of 11.3 nm gold nanoparticles. The role of the weakly physisorbed DMAP ligand in facilitating thermally promoted ripening under mild conditions has also been examined.

Introduction Nanoscale building blocks such as nanoparticles and nanorods are of considerable interest due to their unique size- and shapetunable electronic, magnetic, and catalytic properties, which differ significantly from those of the corresponding bulk-phase material.1,2 In order to incorporate these nanoscale building blocks into functional devices, it is essential to maintain a high degree of control over their individual properties, such as their dimensions, surface charge, and the nature and functionality of the stabilizing ligands used. Advances in the procedures used to prepare nanoparticles, coupled with advances in the ability to incorporate functional moieties onto the nanoparticle surface, have given rise to numerous applications of nanoparticles. These applications include their incorporation into biosensors, as catalysts,8-11 and as materials that can self-assemble into highly ordered structures.12-14 Gold nanoparticles have been the subject of intense research, culminating in preparative techniques in both aqueous15-19 and * Author to whom correspondence should be addressed: fionn.griffin@ esb.ie. (1) Niemeyer, C. Angew. Chem., Int. Ed. 2001, 40, 4128. (2) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (3) Slocik, J. M.; Moore, J. T.; Wright, D. W. Nano Lett. 2002, 2, 169. (4) Obare, S, O.; Hollowell, R. E.; Murphy, C. J. Langmuir 2002, 18, 10407. (5) Qiu, J.; Jiang, X.; Zhu, C.; Shirai, M.; Si, J.; Jiang, N.; Hirao, K. Angew. Chem., Int. Ed. 2004, 43, 2230. (6) Patolsky, F.; Koodali, T.; Ranjit, K. T.; Lichtenstein, A.; Willner, I. Chem. Commun. 2000, 12, 1025. (7) Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (8) Mizukoshi, Y.; Fujimoto, T.; Nagata, Y.; Oshima, R.; Maeda, Y. J. Phys. Chem. B 2000, 104, 6028. (9) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J.; Britz, P.; Brijoux, W.; Bo¨nnemann, H. Langmuir 1997, 13, 2591. (10) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (11) Haruta, M. Catal. Today 1997, 36, 153. (12) Galow, T. H.; Boal, A. K.; Rotello, V. M. AdV. Mater. 2000, 12, 576. (13) Cobbe, S.; Connolly, S.; Ryan, D.; Nagle, L.; Eritja, R.; Fitzmaurice, D. J. Phys. Chem. B 2003, 107, 470. (14) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature (London) 1996, 382, 607. (15) Frens, G. Nature (London), Phys. Sci. 1973, 241, 20. (16) Turkevich, J.; Stevenson, P. C.; Hiller, J. Discuss. Faraday Soc. 1951, 11, 55. (17) Ziegler, K. J.; Doty, R. C.; Johnston, K. P.; Korgel, B. A. J. Am. Chem. Soc. 2001, 123, 7797. (18) de la Fuente, J. M.; Barrientos, A. G.; Rojas, T. C.; Rojo, J.; Can˜ada, J.; Ferna´ndez, A.; Penade´s, S. Angew. Chem. Int. Ed. 2001, 40, 2257. (19) Kanaras, A. G.; Kamounah, F. S.; Schaumburg, K.; Kiely, C. J.; Brust, M. J. Chem. Soc., Chem. Commun. 2002, 2294.

organic media.20-25 Organic solvent-based methods typically incorporate the addition of an organic surfactant, such as alkyl amines or alkylthiols. The function of the surfactant is twofold. It facilitates the transfer of the gold salt from water to an organic phase, in addition to assisting with solubilization of the nanoparticle by adsorbing onto its surface, thereby lowering its surface energy and sterically stabilizing the dispersion. Key nanoparticle parameters, which determine their physical and chemical properties, are their size and shape. A much soughtafter property is the ability to control the size and shape and also the uniformity of the nanoparticles formed. A size-shape monodisperse sample allows the properties of a nanoparticle dispersion to be considered as a summation of the properties of an individual nanoparticle.26-34 Weakly adsorbed stabilizing ligands have been proven to be very attractive as nanoparticle stabilizers due to the ease with which they may be displaced and/or replaced by ligands with tailored functionalities. This opens the possibility of modifying the nanoparticle surface with a variety of ligands, with a variety of receptor groups capable of recognizing another nanoparticle, molecule, or substrate.35 This provides an extra degree of control when attempting to manipulate nanoparticles into desired frameworks. Recently, we have shown that an organic dispersion of gold nanoparticles stabilized by the weakly adsorbed tetra-octylammonium bromide (TOAB) can assemble at the surface of unmodified multiwalled carbon nanotubes (MWCNTs).36 Pseu(20) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (21) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922. (22) Johnson, S. R.; Evans, S. D.; Brydson, R. Langmuir 1998, 14, 6639. (23) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (24) Chen, S.; Sommers, J. M. J. Phys. Chem. B 2001, 105, 8816. (25) Brown, L. O.; Hutchison, J. E. J. Phys. Chem. B 2001, 105, 8911. (26) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316. (27) Stowell, C.; Korgel, B. A. Nano Lett. 2001, 1, 595. (28) Henglein, A.; Meisel, D. Langmuir 1998, 14, 7392. (29) Pileni, M. P. Nat. Mater. 2003, 2, 145. (30) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C.-J. Langmuir 2000, 16, 490. (31) Lin, X. M.; Wang, G. M.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 1999, 103, 5488. (32) Fleming, D. A.; Williams, M. E. Langmuir 2004, 20, 3021. (33) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036. (34) Green, M.; O’Brien, P. Chem. Commun. 2000, 183. (35) Shenhar, R.; Rotello, V. M. Acc. Chem. Res. 2003, 36, 549.

10.1021/la061261a CCC: $37.00 © 2007 American Chemical Society Published on Web 08/23/2007

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dorotaxane recognition chemistry has also been used to assemble appropriately functionalized gold nanoparticles at the surface of functionalized MWCNTs.37 It is with this, and with the adsorption of nanoparticles at other potentially interesting templates such as DNA in mind,38 that this report describes the synthesis of a stable, aqueous dispersion of 4-(dimethylamino)pyridine (DMAP)-stabilized gold nanoparticles. The DMAP adsorbed to the surface of the nanoparticle is bound sufficiently strongly to provide stability to the nanoparticle dispersion while, at the same time, sufficiently weakly to allow phase transfer of the nanoparticles to a thiolcontaining solution of chloroform. Also described is the ability of these nanoparticles to undergo a thermally induced ripening process, whereby their average diameter may be precisely controlled. It is shown that the presence of the unreduced goldDMAP complex is essential to induce thermal ripening of the nanoparticles. Experimental Section Hydrogen tetrachloroaurate (III) trihydrate (ACS Reagent, Aldrich), DMAP (99%, Aldrich), and sodium borohydride (99%, Aldrich) were all used as supplied. All glassware was thoroughly cleaned with aqua regia and was subsequently washed with copious amounts of MilliQ distilled-deionized water (18 MΩ cm). Preparation of Gold Nanoparticle Dispersion 1. Hydrogen tetrachloroaurate (III) trihydrate, (0.150 g, 0.380 mmol) was dissolved in distilled-deionized water (12 mL), producing a bright yellow solution. DMAP (0.250 g, 2.046 mmol) in chloroform (12 mL) was slowly added to this vigorously stirring solution. Upon addition, the mixture became murky brown, turning bright orange after approximately 20 min. Vigorous stirring was continued for 2 h, after which time the phases were separated, with the bright orange aqueous phase being retained. The bright orange aqueous phase was reduced by addition of an aliquot (700 µL, 185 µmol) of a solution of sodium borohydride (0.100 g, 2.64 mmol) in distilled-deionized water (10 mL). The vigorously stirring solution was reduced instantaneously, producing a ruby-red dispersion of DMAP-stabilized gold nanoparticles 1. This dispersion was vigorously stirred for a further 1 h and filtered under reduced pressure to remove any solid material. The recovered DMAPstabilized gold nanoparticles (17.6 µM particle concentration) were fully characterized (see below). Preparation of Gold Nanoparticle Dispersion 2. Reduction using a smaller volume (0.4 mL, 106 µmol) of the borohydride solution produced a dark brown gold nanoparticle dispersion, 2, which was recovered in a similar fashion to 1. The recovered DMAP-stabilized nanoparticles (10.0 µM nanoparticle concentration) were again fully characterized. Phase Transfer of Nanoparticle Dispersions 1 and 2 to Chloroform. Gold nanoparticle dispersions 1 and 2 were transferred to chloroform as follows. Following preparation of 1 or 2, the appropriate gold nanoparticle dispersion was first diluted (2 mL of the stock nanoparticle dispersion was added to distilled-deionized water (10 mL)). This diluted dispersion (12 mL) was added to chloroform (10 mL) containing docecanethiol (DDT, 830 µmol). The two phases were stirred vigorously. After approximately 30 min, the organic phase began to turn brown, indicating phase transfer of the nanoparticles to the organic phase. Continued stirring for approximately 2 h led to complete phase transfer of gold nanoparticle dispersion 1 or 2 to chloroform. The resulting dispersion was characterized by TEM. Thermally Induced Ripening of Nanoparticle Dispersion 2. To thermally ripen the DMAP-stabilized gold nanoparticles, 5 mL of 2 was added to 15 mL of distilled-deionized water. This dilute (36) Fullam, S.; Cottell, D.; Rensmo, H.; Fitzmaurice, D. AdV. Mater. 2000, 12, 1430. (37) Sainsbury, T.; Fitzmaurice, D. Chem. Mater. 2004, 16, 3780. (38) Stanca, S.; Ongaro, A.; Eritja, R.; Fitzmaurice, D. Nanotechnology 2005, 16, 1905.

Langmuir, Vol. 23, No. 20, 2007 10263 dispersion (20 mL) was then heated to 65 °C. Aliquots (200 µL) of the nanoparticle dispersion were extracted at regular time intervals throughout the ripening process and were retained for characterization. After approximately 30 min, the dispersion underwent a color change from brown to ruby-red, indicating an increase in the average diameter of the gold nanoparticles. Heating and sample extraction were continued until the nanoparticle dispersion eventually became unstable and formed a precipitate after approximately 12 h. In a separate experiment, the ripening process was performed under identical conditions to those described above. 60 min into the ripening process, sodium borohydride (300 µL, 80 µmol) was added to the heated nanoparticle dispersion. This reduced any unreduced chloroauric acid-DMAP complex remaining in the ripening dispersion. Heating and sample extraction were continued until the nanoparticle dispersion eventually became unstable and formed a precipitate after approximately 6 h. Thermally Induced Ripening of Nanoparticle Dispersion 1. Immediately after preparation of nanoparticle dispersion 1, 2.5 mL was extracted and added to distilled-deionized water (7.5 mL). The diluted dispersion (10 mL) was then heated in an identical manner to 2. As for 2, samples were extracted at regular time intervals for characterization. Thermally Induced Ripening of Nanoparticle Dispersions 1 and 2 Following Phase Transfer to Chloroform. Phase transfer of nanoparticle dispersion 1 or 2 to a solution of DDT in chloroform led to a chloroformic dispersion of DDT-stabilized gold nanoparticles. These nanoparticles were then subjected to the ripening process under identical conditions to that described previously, with the exception that the temperature was limited to the boiling point of chloroform (62 °C). As before, samples were extracted at regular time intervals for characterization. Thermally Induced Ripening of Nanoparticle Dispersion 1 in the Presence of the Unreduced Chloroauric Acid-DMAP Complex. Immediately after preparation, nanoparticle dispersion 1 was diluted to yield an identical nanoparticle concentration to that present in 2. Following dilution, 2.5 mL was extracted and added to distilled-deionized water (7.5 mL). An aliquot of the bright orange chloroauric acid-DMAP complex (1.88 mL) formed following phase transfer of DMAP to the chloroauric acid-containing aqueous phase was added to the diluted nanoparticle dispersion 1. The amount of unreduced chloroauric acid-DMAP complex added equated to the amount present in nanoparticle dispersion 2 following partial reduction with sodium borohydride. The resulting dispersion was then heated to 65 °C as described above, with samples being extracted at regular time intervals for characterization. Characterization Techniques. All TEM images were recorded using a JEOL JEL-2000 EX electron microscope with a lattice resolution of 0.14 nm and a point-to-point resolution of 0.3 nm operating at 80 kV. All samples, unless stated otherwise, were prepared by evaporating a drop of the appropriate nanoparticle dispersion onto the surface of a carbon-coated 400 square mesh copper TEM grid. The solvent was allowed to evaporate under atmospheric conditions. Optical absorption spectra were recorded at room temperature using a Hewlett-Packard 8452A Diode Array Spectrophotometer. The spectra were recorded over the range 300-820 nm in a quartzglass cuvette with a path length of 1 cm. All spectra were corrected for absorption by the solvent and were normalized to give a measured absorbance close to unity for the size-independent absorption at 440 nm. All proton nuclear magnetic resonance spectra (1H NMR) were recorded using a Varian 300 MHz FT-NMR spectrometer with either the solvent reference or TMS as the internal standard. All chemical shifts are quoted on the δ scale. All coupling constants are expressed in hertz. 1H NMR spectroscopic studies were performed on nanoparticle dispersions prepared in the appropriate deuterated solvent. All mid-infrared spectra (3200 to 1250 cm-1) were recorded using a Matteson Galaxy FT-IR spectrometer. Spectra were recorded against an air background using KBr discs incorporating the required dried sample. All far-infrared spectra (650 to 250 cm-1) were recorded

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using a Perkin-Elmer Spectrum ONE FT-IR spectrometer. Spectra were recorded against a polyethylene background. Samples for measurement were prepared by incorporating the required sample in a disc prepared from spectroscopic grade polyethylene. All gel electrophoresis studies were performed in a 2% agarose gel matrix. All zeta potential measurements were performed using a Malvern Zetasizer 3000 HSA at a predetermined pH and were calculated using the Smoluchowski approximation.

Results and Discussion Preparation and Characterization of Nanoparticle Dispersions 1 and 2. This report describes a method for the preparation of an aqueous dispersion of DMAP-stabilized gold nanoparticles. The procedure used is based on that reported by Brust et al. for the production of gold nanoparticles stabilized by alkylthiol molecules in an organic phase.20,21 A chloroformic solution of DMAP is added to an aqueous solution of the chloroauric acid, resulting in phase transfer of DMAP to the aqueous phase, forming a bright orange solution. The DMAP remaining in the chloroform phase was recovered and, having being identified by 1H NMR, was weighed. It was found that only 3% (7.5 mg) of the added DMAP was retained in the chloroformic phase; i.e., 97% was transferred to the aqueous phase. It should be noted that the transfer of DMAP from the chloroformic phase to the aqueous phase is accompanied by the appearance of a bright orange color in the aqueous phase. In a control experiment, vigorous stirring of a chloroformic solution of DMAP with distilled-deionized water (in the absence of the chloroauric acid) resulted in a negligible phase transfer of DMAP to the aqueous phase (less than 1%) and no color change. The ability of pyridine and its derivatives to complex with chloroauric acid by the displacement of chlorine leading to the formation of complexes such as [AuCl3(py)] and [AuCl2(py)2]+ has been reported.39,40 These substitution reactions are reversible, and the pyridine interacts weakly with the central gold atom. Of particular relevance is the fact that the rate of coordination increases with increasing pKa of the substituting ligand. DMAP has a relatively high pKa of 9.7, and therefore coordination of DMAP with chloroauric acid upon phase transfer is expected to be rapid. Consistent with this view is the rapid color change from yellow to brown to bright orange observed upon addition of DMAP to the chloroauric acid-containing aqueous phase during preparation of dispersions 1 and 2. NMR supports the view that DMAP complexes with chloroauric acid. Figure 1a shows the 1H NMR spectrum of DMAP in water. Figure 1b shows the 1H NMR spectrum following phase transfer of DMAP from the chloroformic phase to the aqueous phase containing chloroauric acid. The ratio of chloroauric acid to DMAP in this sample is 1.0:5.4. The symmetrical protons adjacent to the pyridine nitrogen are labeled R, while the symmetrical protons adjacent to these are labeled β. It is noted that the resonances assigned to the R and β protons of the pyridine ring in the 1H NMR spectrum in Figure 1b are split, suggesting the existence of multiple forms of DMAP in solution. This is consistent with the ability of DMAP to undergo similar substitution reactions with chloroauric acid to those reported for functionalized pyridine molecules.39,40 Thus, it may be concluded that a significant quantity of the DMAP transferred to the aqueous phase is complexed with the gold salt, while the remainder exists free in solution. In short, it appears that the transfer of DMAP from the chloroformic phase to the aqueous phase requires the presence (39) Adams, H.; Stra¨hle, J. Z. Anorg. Allg. Chem. 1982, 485, 65. (40) Cattalini, L.; Nicolini, M.; Orio, A. Inorg. Chem. 1966, 5, 1674.

Figure 1. 1H NMR spectra of (a) DMAP and (b) the unreduced chloroauric acid-DMAP complex, formed following transfer of DMAP from the chloroformic phase to the chloroauric acid containing the aqueous phase.

Figure 2. (a) Transmission electron micrograph of the DMAPstabilized gold nanoparticle dispersion 1. Inset shows the microelectron diffraction pattern of the nanoparticles. (b) Histogram of the diameters of 226 nanoparticles with an average diameter of 3.6 ( 0.5 nm.

of the chloroauric acid in the aqueous phase and that DMAP complexes with the chloroauric acid to produce a color change. Following recovery and reduction of the aqueous phase by addition of a dilute solution (185 µmol) of sodium borohydride, a stable dispersion of DMAP-stabilized gold nanoparticle dispersion 1 was obtained. A TEM image of the nanoparticles formed is shown in Figure 2a. The nanoparticles possess an average diameter of 3.6 ( 0.5 nm (calculated from the diameters of a sample of 226 nanoparticles); see Figure 2b. The characteristic surface plasmon band for gold at 520 nm was observed in the optical absorption spectrum of dispersion 1; see Figure 3. The crystallinity of the nanoparticles formed was established by examining the corresponding microelectron diffraction pattern. Using evaporated thallous chloride (Agar Scientific) as a reference, the diffraction pattern shown in Figure 2a (inset) was obtained. The rings in the polycrystalline diffraction pattern

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Figure 3. Absorption spectra of DMAP-stabilized gold nanoparticle dispersions (a) 1 and (b) 2.

indicate the presence of face-centered cubic (fcc) gold nanoparticles and are assigned to the {111}, {200}, {220}, and {311} planes. The presence of DMAP on the surface of the nanoparticles in dispersion 1 was confirmed by 1H NMR. 1,4-Dioxane was used as an internal standard to quantify the amount of DMAP present in solution, both prior to and following reduction of the chloroauric acid. Following reduction, the amount of DMAP detected decreased. Since DMAP adsorbed at the surface of a gold nanoparticle is not detected by NMR, the difference was assumed to correspond to the amount of DMAP adsorbed at the nanoparticle surface.41,42 On this basis, it was estimated that there are, on average, 242 DMAP molecules adsorbed at the surface of each nanoparticle in dispersion 1. This corresponds to an average of 17 Å2 per DMAP molecule. This value is consistent with that observed for close-packed pyridine molecules adsorbed normal to the surface of a positively charged Au(111) electrode.43 In order to determine the surface charge of the DMAP-stabilized nanoparticles in dispersion 1, gel electrophoresis and microelectrophoresis (zeta potential) measurements were performed. Gel electrophoresis, performed in an agarose gel matrix, resulted in migration of the nanoparticles toward the negative electrode, indicating a net positive charge on the nanoparticles. Microelectrophoresis performed on a dilute nanoparticle dispersion (1.76 nmol) immediately after preparation gave an average value of +40.7 mV at pH 8.0. This finding confirms that these particles possess an overall positive charge at the radius of shear at this pH and accounts for the stability of the nanoparticles over extended periods of time. The nanoparticles prepared by reduction with a lesser amount of sodium borohydride (106 µmol) resulted in nanoparticle dispersion 2. The amount of borohydride used was insufficient to reduce all of the chloroauric acid present to elemental gold. Accordingly, the dispersion contains elemental gold in the form of nanoparticles, in addition to gold present as a chloroauric acid, either complexed with DMAP or free in solution. A TEM image of nanoparticle dispersion 2 is shown in Figure 4a. TEM analysis of the diameters of 243 nanoparticles yielded an average diameter of 3.9 nm. Figure 4b shows the size distribution for the nanoparticles. These nanoparticles are reasonably polydisperse, with diameters ranging from 1.6 to 6.5 nm. It is noted, however, that of the 241 nanoparticles analyzed, the majority (97%) were in the 1.6-4.9 nm range. Analysis of the corresponding (41) Terrill, R.; Postlethwaite, T.; Chen, C.; Poon, C.; Terzis, A.; Chen, A.; Hutchison, J.; Clark, R.; Wignall, G.; Londono, J.; Superfine, R.; Falvo, M.; Johnson, C.; Samulski, E.; Murray, R. J. Am. Chem. Soc. 1995, 117, 12537. (42) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G.; Lennox, R. Chem.sEur. J. 1996, 2, 359. (43) Lipkowski, J.; Stolberg, L. Adsorption of Molecules at Metal Electrodes; VCH: New York, 1992.

Figure 4. (a) Transmission electron micrograph of the DMAPstabilized gold nanoparticle dispersion 2. Inset shows the microelectron diffraction pattern of the nanoparticles. (b) Histogram of the diameters of 243 nanoparticles with an average diameter of 3.9 ( 1.6 nm (for 97% of nanoparticles in the 1.6 nm to 4.9 nm diameter range).

microelectron diffraction pattern (Figure 4a inset) shows that, as for nanoparticle dispersion 1, the rings correspond to fcc gold. In addition, the surface plasmon band for nanoparticle dispersion 2 is broader than that for nanoparticle dispersion 1; see Figure 3. The amount of DMAP adsorbed to the surface of each nanoparticle in dispersion 2 was calculated by 1H NMR analysis of the dispersion before and after reduction as described for dispersion 1. It was estimated that there are 460 DMAP molecules adsorbed to the surface of each nanoparticle. The increase in this figure compared to that for dispersion 1 may be partially accounted for by the increased surface area of the nanoparticles in dispersion 2. This corresponds to an average area per DMAP molecule of about 10.4 Å2, a value lower than that expected on the basis of reports for close-packed pyridine molecules adsorbed normal to the surface of a positively charges Au(111) gold electrode.43 As discussed below, this higher figure may be due to interaction of the unreduced chloroauric acid-DMAP complex with the surface of the nanoparticle. The zeta potential of dispersion 2, calculated immediately after preparation, was +26.7 mV for measurements made at pH 8.0. As for dispersion 1, the nanoparticles possess a net positive charge at the radius of shear and are stable, albeit not as stable as dispersion 1 (+40.7 mV). 1H NMR Characterization of Nanoparticle Dispersions 1 and 2. The 1H NMR of an aqueous solution of DMAP is shown in Figure 1a. The resonances assigned to the R and β protons of free DMAP appear as doublets at δ 7.94 and δ 6.73, respectively. The 1H NMR spectrum obtained for nanoparticle dispersion 1 is shown in Figure 5a and is in close agreement with the spectrum in Figure 1a. The resonances at δ 7.93 and δ 6.80 in the 1H NMR spectrum of dispersion 1 are assigned to the R and β pyridine protons of free DMAP. The downfield shift of the resonance assigned to the β proton, relative to the resonance assigned to the same proton in Figure 1a, is due to the different pH of the two samples. As expected, there are no resonances that can be

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Figure 5. 1H NMR spectra of (a) the dispersion of fully reduced DMAP-stabilized gold nanoparticles 1 and (b) a dispersion of partially reduced DMAP-stabilized gold nanoparticles 2.

assigned to DMAP complexed with chloroauric acid, since sufficient reducing agent was added to ensure all the chloroauric acid initially present was reduced to elemental gold. Also, as expected, there are no resonances that can be assigned to DMAP adsorbed at the surface of the nanoparticles in dispersion 1, since molecules adsorbed at the surface of gold nanoparticles are not detectible by NMR spectroscopy.41,42 The 1H NMR of an aqueous solution of DMAP and chloroauric acid prior to reduction is shown in Figure 1b. The resonances assigned to the R and β protons of free and complexed DMAP appear as a series of doublets between δ 8.2 and δ 7.7 and between δ 6.9 and δ 6.4, respectively. The spectrum obtained for dispersion 2, shown in Figure 5b, is in close agreement with that in Figure 1b. The resonances assigned to the R and β protons of free and complexed DMAP appear as series of doublets between δ 8.0 and δ 7.7 and between δ 6.9 and δ 6.6, respectively. These resonances are assigned to a both free and complexed DMAP, since insufficient reducing agent was added to ensure all the chloroauric acid initially present was reduced. Also, as expected, there are no resonances that can be assigned to DMAP adsorbed at the surface of the nanoparticles in dispersion 2, since molecules adsorbed at the surface of gold nanoparticles are not detectible by NMR spectroscopy.41,42 Far- and Mid-Infrared Characterization of Nanoparticle Dispersions 1 and 2. In order to obtain further insight into the nature of the interaction of DMAP with chloroauric acid and with the gold nanoparticle surface, far- (650-250 cm-1) and mid- (3200-1200 cm-1) infrared spectra were recorded for the following samples: KAuCl4 (used in the place of HAuCl4‚3H2O due to the absence of water of crystallization and its reduced hydroscopic properties); DMAP; an aqueous solution of DMAP and chloroauric acid (2.7:1.0); an aqueous solution of DMAP and chloroauric acid (5.4:1.0); the gold nanoparticle dispersion 2; and the gold nanoparticle dispersion 1. These spectra are shown in the Supporting Information. The far-infrared spectrum of KAuCl4 is shown in Figure 6a and is in good agreement with reported spectra.44 The band at 354 cm-1 is assigned to the Au-Cl stretch. The far-infrared spectrum of DMAP is also in good agreement with reported reference spectra45 and is shown in Figure 6b. The bands at 540 and 530 cm-1 are assigned to the pyridine ring in-plane deformation, while the band at 479 cm-1 is assigned to the pyridine out-of-plane ring deformation.46,47 (44) Sabatini, A.; Sacconi, L.; Schettino, V. Inorg. Chem. 1964, 3, 1775. (45) The Aldrich Library of Infrared Spectra, 3rd ed.; Aldrich Chemical Company Inc.: 1981.

Figure 6. Far-IR spectra of (a) KAuCl4, (b) DMAP, (c) a 1:2.7 mixture of chloroauric acid and DMAP, (d) a 1:5.4 mixture of chloroauric acid and DMAP, (e) the DMAP-stabilized gold nanoparticle dispersion 2, and (f) the DMAP-stabilized gold nanoparticle dispersion 1.

The spectrum recorded following transfer of DMAP from the chloroformic phase to the aqueous phase containing chloroauric acid is shown in Figure 6c. The ratio of DMAP to chloroauric acid is 2.7:1. At this ratio, there is insufficient DMAP present to completely substitute all the chlorine atoms present in the chloroauric acid. Hence, partial substitution of chlorine by DMAP occurs, resulting in a decrease in the symmetry of the square planar chloroauric acid.44 This is manifest as a splitting of the band at 354 cm-1 assigned to the Au-Cl stretch. Furthermore, the bands at 540 and 530 cm-1 and the band at 479 cm-1 assigned to the in-plane and out-of-plane deformations of the pyridine ring, respectively, are replaced by a series of overlapping bands between 540 and 470 cm-1 that are assigned to complexed pyridine. In addition, a new series of overlapping bands between 290 and 270 cm-1 are observed that are assigned to the Au-N stretch of the complexed pyridine molecule.46,47 The spectrum recorded following transfer of DMAP from the chloroformic phase to the aqueous phase containing chloroauric acid is shown in Figure 6d. The ratio of DMAP to chloroauric acid is 5.4:1. As might be expected, the band assigned to the Au-Cl stretch at 354 cm-1 in Figure 6a has been replaced by a series of relatively weak and overlapping bands. This is consistent with the substitution of the majority of the chlorines in the chloroauric acid by DMAP. Figure 6e,f shows the far-infrared spectra of nanoparticle dispersions 2 and 1, respectively. The band assigned to the AuCl stretch in chloroauric acid is almost completely absent from Figure 6e. This is as expected for two reasons. First, approximately 60% of the chloroauric acid has been reduced to elemental gold. Second, the unreduced chloroauric acid is complexed with DMAP, and the majority of the chlorine atoms have been displaced. Figure 6f shows the far-infrared spectrum for dispersion 1. This dispersion has been fully reduced, and therefore only contains elemental gold in the form of a gold nanoparticle. As expected, there is no band visible in the region of the Au-Cl stretch. In (46) Bellamy, L. The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: London, 1975. (47) Roeges, N. A Guide to the Complete Interpretation of Infrared Spectra of Organic Structures; Wiley: New York, 1998.

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Scheme 1

both Figure 6e and f there are bands between 290 and 270 cm-1. As above, these bands are assigned to the Au-N stretch of a pyridine molecule adsorbed at the surface of a gold nanoparticle or complexed with chloroauric acid.46,47 The overlapping series of bands assigned to the in-plane and out-of-plane deformations of the pyridine ring between 540 and 480 cm-1 are once again observed in Figure 6e,f. However, it is clear that the bands at the frequencies corresponding to free DMAP; specifically, the bands at 540, 530, and 479 cm-1 are more intense for dispersion 1 than for dispersion 2. This is consistent with complete reduction of the chloroauric acidDMAP complex to form the gold nanoparticles in dispersion 1, and as a consequence, a larger fraction of the DMAP is expected to be free in solution. The mid-infrared spectra of the samples described in the farinfrared region above were also analyzed (see Supporting Information). A detailed analysis of these spectra is outside the scope of this report. It is noted however that bands are observed for DMAP in the presence of chloroauric acid or in the presence of gold nanoparticles at 3150 and 3069 cm-1 that are assigned to a vinylic C-H stretch.46,47 Similarly, a band observed at 1645 cm-1 is assigned to an aliphatic CdC stretch. The interaction of pyridine with gold surfaces can occur via the delocalized π orbitals on the pyridine ring (π-bonded) or via the lone pair of electrons residing on the cyclic nitrogen (Nbonded). The nature of the interaction depends on the charge and on the crystal structure of the metal nanoparticle. For positively charged Au(111), the pyridine is adsorbed through the nitrogen atom.48,49 An analysis of the projected density of states (PDOS) has shown the interaction to be noncovalent, and due to interaction of the N pz orbital and the Au dz2 orbital.50 Since the DMAPstabilized nanoparticle dispersions 1 and 2 possess a net positive charge and a suitable crystal structure, it is proposed that DMAP is adsorbed to the gold nanoparticle surface through the nitrogen atom of the pyridine ring; see Scheme 1. The presence of bands that are assigned to a vinylic CsH stretch and an aliphatic CdC stretch support the view that DMAP interacts with the gold nanoparticle surface and with [AuCl4]- via the pyridine nitrogen as shown in Scheme 2. Donation of electron density from the pyridine nitrogen pz orbital to the dz2 orbital of gold results in the aromatic character of the molecule being reduced, leading to a conjugated amine, confirmed by the presence of the above bands in the measured spectra. Strength of the Interaction of DMAP with the Nanoparticle Surface. Straight-chain aliphatic amines have been shown to passivate the surface of gold nanoparticles; however, the interaction with the gold surface is relatively weak.51 Caruso et al. have shown that tetraoctylammonium bromide (TOAB)stabilized gold nanoparticles can be transferred from an organic phase to a DMAP-containing aqueous phase.52 (48) Hoon-Kholsa, M.; Fawcett, W.; Chen, A.; Lipkowski, J.; Pettinger, B. Electrochim. Acta 1999, 45, 611. (49) Cai, W.; Wan, L.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992. (50) Bilic´, A.; Reimers, J.; Hush, N. J. Phys. Chem. B 2002, 106, 6740. (51) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2003, 15, 935. (52) Gittins, D.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001.

Figure 7. (a) Transmission electron micrograph of DDT-stabilized gold nanoparticles prepared by displacement of DMAP from the surface of the DMAP-stabilized gold nanoparticles in dispersion 1 and their transfer to the chloroform phase. (b) Histogram of the diameters of 214 nanoparticles. The nanoparticles possess an average diameter of 4.1 ( 0.4 nm.

On this basis, it is concluded that DMAP molecules are weakly physisorbed at the surface of the gold nanoparticles. To confirm this, nanoparticle dispersion 1 was washed multiple times with chloroform, which resulted in DMAP being extracted from the aqueous phase into the chloroform phase. 1H NMR analysis of the chloroform confirms the extraction of DMAP. After multiple washings, the DMAP-stabilized gold nanoparticles become destabilized and precipitate. The relative ease with which the DMAP is removed from the surface of the gold nanoparticles precludes strong chemisorption of the DMAP. Furthermore, it is found that vigorously stirring a diluted sample of the nanoparticle dispersion 1 or 2 (2 mL of 1 or 2 added to 10 mL of distilled-deionized water) in the presence of chloroform (10 mL) containing DDT (830 µmol) results in the transfer of the nanoparticles to the chloroformic phase. As can be seen from Figure 7a, the DDT-stabilized gold nanoparticles derived from dispersion 1 maintain their integrity in the chloroformic phase, possessing an average diameter of 4.1 ( 0.4 nm. The corresponding microelectron diffraction pattern, Figure 7a (inset), confirms that these nanocrystals possess an fcc crystal structure. A similar finding is reported for gold nanoparticle dispersion 2, yielding 3.9 ( 0.4 nm DDT-stabilized gold nanoparticles with fcc crystallinity. Thermally Promoted Ripening of Nanoparticle Dispersion 2. In preparing nanoparticle dispersion 2, the unreduced chloroauric acid-DMAP complex formed following phase transfer was reduced with 106 µmol of sodium borohydride. As mentioned, the number of electrons provided by the borohydride is insufficient to completely reduce all of chloroauric acid present to elemental gold. Following preparation of dispersion 2, a sample (5 mL) was extracted, diluted in deionized-distilled water (15 mL), and heated to 65 °C. The thermally induced ripening of the nanoparticles was monitored as follows: changes in the nanoparticle diameter were determined by TEM, while changes in the

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Figure 8. (a) Transmission electron micrograph of the DMAPstabilized gold nanoparticle dispersion 2 as sampled 60 min into the thermally induced ripening process at 65° C. Inset shows the microelectron diffraction pattern for the nanoparticles. (b) Histogram of the diameters of 181 ripened nanoparticles. The size distribution is significantly broadened.

corresponding optical absorption spectra were monitored by optical absorption spectroscopy. The nanoparticles in dispersion 2 possess an initial average diameter of 3.9 nm; see Figure 4a. A TEM image of the nanoparticles extracted after heating for 60 min shows a significantly broadened distribution of nanoparticle diameters; see Figure 8a. A histogram of 181 nanoparticle diameters is shown in Figure 8b. The ripened gold nanoparticles retain their fcc crystal structure (Figure 8a inset) and have an average diameter of 7.5 nm. At this stage of the ripening process, the nanoparticles exist as a dispersion containing a mixture of small (∼4 nm) and large (∼8 nm) diameters. Continued heating of nanoparticle dispersion 2 for a total of 570 min results in an increase in the average nanoparticle diameter and a narrowing of the distribution of nanoparticle diameters; see Figure 9a. The final dispersion is composed of nanoparticles with an average diameter of 11.3 ( 1.5 nm (calculated for a sample of 205 nanoparticle diameters); see Figure 9b. As a consequence of their relative monodispersity, these nanoparticles exhibit a greater tendency to form self-assembled monolayers and bilayers on carbon-coated TEM grids. The average zeta potential of the ripened nanoparticles is +39.2 mV. This value is higher than that for nanoparticle dispersion 2 (+26.7 mV), which implies increased stability of the dispersion composed of the ripened nanoparticles. As will be discussed below, the increased stability may be due to the reduction in the concentration of the unreduced chloroauric-DMAP present following ripening of the nanoparticles. It is well-established that the intensity of the band assigned to the surface plasmon in the optical absorption spectrum of a gold nanoparticle dispersion increases as the average diameter of the nanoparticles increases.53,54 It has also been established, (53) Alvarez, M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (54) Kreibig, U.; Genzel, L. Surf. Sci. 1985, 156, 678.

Griffin and Fitzmaurice

Figure 9. (a) Transmission electron micrograph of the DMAPstabilized gold nanoparticle dispersion 2 as sampled 570 min into the thermally induced ripening process at 65°C. Inset shows the microelectron diffraction pattern for the nanoparticles. (b) Histogram of the diameters of 205 ripened nanoparticles. The nanoparticles possess an average diameter of 11.3 ( 1.5 nm.

in accordance with Mie theory, that the wavelength of maximum absorption of the surface plasmon band is independent of the average diameter of nanoparticles between 3 and 20 nm.55,56 Figure 10a shows the optical absorption spectra of dilute dispersions prepared from nanoparticles extracted from dispersion 2 at various times during the ripening process. All spectra have been normalized with respect to the size-independent absorbance at 440 nm.54 The spectrum for the as-prepared nanoparticles (t ) 0 min), shows the expected broad surface plasmon band centered at 520 nm. As the thermal ripening proceeds and the average nanoparticle diameter increases, there is an increase in the intensity of this band and an associated color change. After 570 min, the nanoparticles have ripened into a dispersion composed of nanoparticles possessing an average diameter of 11.3 nm. This dispersion is stable indefinitely under atmospheric conditions at room temperature. Heating for extended periods of time results in eventual precipitation of the nanoparticle dispersion. This is characterized by a dramatic red-shift in the surface plasmon maximum to 634 nm after 700 min. Figure 10b is a plot of the difference between the absorbance measured at time t, (At) and the initial absorbance (A0) against time, and clearly shows that the evolution process occurs rapidly initially. Approximately 200 min into the ripening process, the plot begins to plateau, indicating a decrease in the rate of nanoparticle growth. Once this point has been reached, the increase in the average diameters of the nanoparticle dispersion is more gradual. TEM supports these observations; average diameters of nanoparticles extracted at 270, 360, and 570 min into the ripening process vary only slightly (10.9, 11.1, and 11.3 nm, respectively). This observation is supported by slight increases in intensity in the optical absorption spectra as time proceeds, compared to early into the ripening process. (55) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (56) Doremus, R. H. J. Chem. Phys. 1964, 40, 2389.

Stabilized Water-Soluble Gold Nanoparticles

Figure 10. (a) Absorption spectra of DMAP-stabilized gold nanoparticle dispersion 2 extracted at the indicated time during the ripening process. The spectrum at t ) 0 min is that for the unripened DMAP-stabilized gold nanoparticle dispersion, 2. The spectrum at t ) 700 min, red-shifted to 634 nm, is that for the destabilized nanoparticle dispersion. (b) Plot of (At - A0) at 520 nm versus time elapsed during the thermally induced ripening process.

Determination of the Role of the Unreduced Chloroauric Acid-DMAP Complex in the Ripening Process. It is important to establish whether the unreduced chloroauric acid, either complexed to DMAP or free in solution, plays a role in the ripening process. Accordingly, the process of thermal ripening was repeated in D2O. Samples were extracted every 60 min for 1H NMR analysis to which a known quantity of 1,4-dioxane (1.2 µmol) was added as an internal standard. It was found that the concentration of choloroauric acid complexed to free DMAP in solution decreased by 60%. It is assumed that this gold is incorporated into the ripening nanoparticles. However, if this were the only mechanism for nanoparticle growth, an increase in the average diameter of the nanoparticles present from 3.9 to 4.4 nm would be observed. Since the nanoparticles grow to a final diameter of 11.3 nm, it may be concluded that nanoparticle growth is not exclusively due to the incorporation of the choloroauric acid-DMAP complex. The 1H NMR spectrum of the final ripened DMAP-stabilized nanoparticles shows that a fraction of the unreduced choloroauric acid-DMAP complex remains in the solution following ripening of the nanoparticles (see Supporting Information). In order to further determine the role of the unreduced chloroauric acid, either complexed to DMAP or free in solution, a series of additional experiments were undertaken. The first experiment involved heating the as-prepared nanoparticle dispersion 1 to 65 °C in an identical manner to that used to ripen nanoparticle dispersion 2. However, nanoparticle dispersion 1 did not ripen, and no increase in nanoparticle diameter was observed. It is noted that this dispersion does not contain

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any unreduced chloroauric acid, either complexed to DMAP or free in solution. In two separate experiments, attempts to ripen the DDTstabilized gold nanoparticles in chloroform (formed by phase transfer of the DMAP-stabilized gold nanoparticle dispersions 1 and 2) failed. As for dispersion 1 above, the unreduced chloroauric acid-DMAP complex is absent from the DDTstabilized gold nanoparticles prepared from dispersions 1 and 2. An additional quenching experiment was performed on nanoparticle dispersion 2. The dispersion was subjected to the ripening conditions as described previously. After 60 min, ripening had commenced, as determined by an increase in the intensity of the surface plasmon band for a sample extracted at this time. Addition of sufficient sodium borohydride at this time (60 min into the ripening process) to completely reduce the unreduced chloroauric acid-DMAP complex present to elemental gold led to a quenching of the ripening process, and no further ripening was observed by TEM or by optical absorption spectroscopy. After 360 min, the dispersion became destabilized and precipitated. Thus, since these four systems failed to undergo thermally induced ripening, it may be concluded that the unreduced chloroauric acid-DMAP complex does indeed play a vital role in the growth mechanism, since ripening is only observed in its presence. The above conclusion is further supported by the observation that the nanoparticle dispersion 1 undergoes thermally induced ripening following addition of the unreduced chloroauric acidDMAP complex to the ripening dispersion. Addition of an identical amount of the unreduced chloroauric acid-DMAP complex to dispersion 1 as is present in dispersion 2 prior to ripening leads to growth in the average nanoparticle diameter over time. As mentioned previously, dispersion 1 is more stable than dispersion 2 (1 has a higher zeta potential than 2). However, it should be noted that the zeta potential of dispersion 1 is reduced from +40.7 to +14.2 mV upon addition of the unreduced chloroauric acid-DMAP complex, which may be due to interaction of the complex with the nanoparticle surface. It is found that dispersion 1 evolves from an average diameter of 3.6 nm to a final average diameter of 7.7 nm. It is also noted that, during the ripening process, dispersion 1 undergoes a transition from an initial monodisperse sample to a dispersion with increased size distribution at t ) 60 min. This is a similar feature to that observed for dispersion 2, which also became polydisperse 60 min into the ripening process. The average diameter of the final ripened nanoparticles at t ) 750 min is 7.7 ( 2.5 nm (see Supporting Information). Figure 11a shows the optical absorption spectra of dilute dispersions prepared from nanoparticles extracted at various time intervals throughout the ripening process, clearly showing an increase in the intensity and a sharpening of the surface plasmon band over time. A similar observation was noted in the optical absorption spectra of the ripening nanoparticle dispersion 2. Addition of ionic species is known to induce nanoparticle aggregation, which would be accompanied by a red-shift in the surface plasmon band. However, since no red-shift is observed upon addition of the unreduced chloroauric acid-DMAP complex to dispersion 1, nanoparticle growth by aggregation may be ruled out. Interestingly, performing the ripening of dispersion 1, containing an identical number of nanoparticles and an identical amount of the unreduced chloroauric acid-DMAP complex as was present for dispersion 2, shows similar kinetics to those of the ripening process; see Figure 11b.

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Figure 11. (a) Absorption spectra of DMAP-stabilized gold nanoparticle dispersion 1 extracted at the indicated time during the ripening process. The spectrum at t ) 0 min is that for the unripened DMAP-stabilized gold nanoparticle dispersion 1 in the presence of the unreduced chloroauric acid-DMAP complex. (b) Plot of (At A0) at 520 nm versus time elapsed during the thermally induced ripening process.

Mechanism for Thermally Promoted Ripening. The ability of gold nanoparticles to undergo a ripening process is not a new phenomenon and has been discussed in detail by many workers for various nanoparticle systems.30,31,57,58 Zhong and co-workers have demonstrated the ability of a polydisperse dispersion of DDT-stabilized gold nanoparticles to ripen into a dispersion with an increased diameter and a significantly narrower size distribution.30 The ability of large polyhedral gold nanoparticles stabilized by dodecyldimethylammonium bromide (DDAB) to undergo a digestive ripening process when heated in the presence of DDT, resulting in a reasonably monodisperse dispersion of 2-6 nm nanoparticles, has also been demonstrated.31 These procedures are performed in organic media and do not involve the presence of an unreduced gold-ligand complex in the ripening nanoparticle dispersions. The driving force for nanoparticle evolution is related to the surface energies of the nanoparticles involved. Due to their higher radius of curvature, smaller nanoparticles possess a higher surface energy than larger nanoparticles of similar composition. Smaller nanoparticles also have a larger surface area to volume ratio than larger nanoparticles, and are therefore more soluble than larger nanoparticles. This observation is the basis for the GibbsThomson equation and may be expressed as

Sr ) Sb exp(2σVm/rRT)

(1)

(57) Jin, R.; Cao, C.; Hao, E.; Me´traux, G. S.; Mirkin, C. A. Nature (London) 2003, 425, 487. (58) Madras, G.; McCoy, B. J. J. Chem. Phys. 2003, 119, 1683.

where Sr and Sb are the respective solubilities of particles of radius r and the bulk material, σ represents the particles surface energy, Vm is the molar volume of the particle, R is the gas constant, and T is the absolute temperature. The larger nanoparticles increase in diameter at the expense of the more soluble smaller nanoparticles. The rate of this ripening process, termed Ostwald ripening, decreases over time as the average particle size increases. This is a consequence of a decrease in the difference in solubility between nanoparticles and in the decreased frequency with which the dissolved atoms or molecules encounter another nanoparticle in solution. Ostwald ripening is generally associated with a narrowing of the nanoparticle size distribution and has been considered in detail by Lifshitz and Sloyzof and also by Wagner (LSW theory).59,60 It is within this physical framework that the thermally induced ripening of the DMAP-stabilized gold nanoparticles described here is discussed. The experimental evidence described above clearly shows that the unreduced chloroauric acid-DMAP complex is essential for thermally promoted ripening to occur under the described conditions. However, the question remains as to the mechanism of the ripening process. During the nanoparticle synthesis, reduction of the unreduced chloroauric acid-DMAP complex by addition of sodium borohydride changes the free energy of the system, leading to phase separation of the components and the formation of elemental gold nuclei. The formation of these nuclei depletes the amount of gold monomer units present, leading to further growth of nanoparticles by addition of monomer units to the nuclei formed. Once all of the monomer units have been consumed, nanoparticle growth stops. Adsorption of DMAP to the surface of the nanoparticles also limits further growth. Hence, the number of nanoparticles per unit volume is larger for dispersion 1 than for dispersion 2, since complete reduction of all the chloroauric acid-DMAP complex present occurs. The presence of adsorbed ions at the nanoparticle electrical double layer are known to reduce, and even reverse, the sign of the zeta potential at sufficiently high concentration.61 This is consistent with the observed lowering of the zeta potential of dispersion 1 when the unreduced chloroauric acid-DMAP complex is added. In this context, it is noted that the chemisorption of ions at the surface of both silver and gold nanoparticles has been previously reported.62-64 Wang and co-workers have reported the adsorption of iodide onto the surface of citratestabilized gold nanoparticles. The iodide displaces citrate from the nanoparticle surface, resulting in a lowering of the zeta potential, leading to an increase in the attractive force between the nanoparticles, which in turn is responsible for the formation of larger nanoparticle aggregates. The authors propose that the aggregates are formed via an Ostwald ripening process, and that addition of KI to the gold nanoparticles enhances the rate of ripening. In addition to the reduction of the zeta potential, adsorption of iodide leads to a dampening (broadening) and an eventual red-shift of the surface plasmon band. Control experiments using chloride ions failed to induce nanoparticle aggregation. Hence, while nanoparticle dispersions 1 and 2 contain the same amount of chloride ions, the chloroauric acid-DMAP complex is only present in nanoparticle dispersion 2. It may be concluded therefore that the chloroauric acid-DMAP complex (59) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35. (60) Wagner, C. Z. Electrochem. 1961, 65, 581. (61) Hunter, R. J. Zeta Potential in Colloid Science Principles and Applications; Academic Press Inc.: London, 1981. (62) Linnert, T.; Mulvaney, P.; Henglein, A. J. Phys. Chem. 1993, 97, 679. (63) Cheng, W.; Dong, S.; Wang, E. Angew. Chem., Int. Ed. 2003, 42, 449. (64) Pal, T.; Jana, N. R.; Sau, T. K. Corros. Sci. 1997, 39, 981.

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interact with the nanoparticle surface affects the surface energy of the nanoparticles. In this instance, the interaction of the chloroauric acid-DMAP complex with the DMAP-stabilized gold nanoparticles increases the surface energy of the nanoparticles, enhancing the process of removal of gold atoms from the smaller nanoparticles, which then diffuse to and adsorb onto the larger nanoparticles. The interaction of the chloroauric acidDMAP complex with the nanoparticle surface also accounts for the higher number of DMAP molecules on the surface of nanoparticle dispersion 2 compared to dispersion 1.

Conclusions

Figure 12. Energy profile diagram, illustrating the increase in the overall energy of nanoparticle dispersion 1 in (I) the absence and (II) in the presence of the unreduced chloroauric acid-DMAP complex. The energy barrier is too high for I to ripen at 65 °C. The subsequent energy increase of the system, in addition to the elevated temperature, facilitates Ostwald ripening into larger, more thermodynamically stable nanoparticles.

is responsible for the lower zeta potential and the broader surface plasmon band for nanoparticle dispersion 2 compared to nanoparticle dispersion 1 (see Figure 3). On the basis of our own and other reported findings, it is proposed that the surfaces of the DMAP-stabilized gold nanoparticles are modified by their interaction with the unreduced chloroauric acid-DMAP complex. The fact that the thermally promoted ripening of the nanoparticles only occurs in its presence would imply that the surface energy of the nanoparticles is increased upon interacting with the complex, which in turn implies an increase in nanoparticle solubility; see eq 1. The increase in the surface energy and solubility of the nanoparticles lowers the activation energy barrier to nanoparticle dissolution, and growth is observed at only moderately elevated temperatures, i.e., Ostwald ripening. The proposed mechanism is illustrated in Figure 12, which shows an energy profile diagram for the ripening of dispersion 1 in the absence (I) and in the presence (II) of the unreduced chloroauric acid-DMAP complex. System I in Figure 12 represents the energy profile curve for nanoparticle dispersion 1, which does not ripen at 65 °C. Scheme II in Figure 12 represents the energy profile curve for nanoparticle dispersion 2 or nanoparticle dispersion 1 in the presence of the unreduced chloroauric acid-DMAP complex, both of which undergo ripening to larger nanoparticles at moderately elevated temperatures. The presence of the complex lowers the barrier to ripening by increasing the energy of the nanoparticles. It should be noted that an analogous effect as been observed by Jana and co-workers, who described the complete reversible dissolution and reformation of silver nanoparticles in the presence of a nucleophile (borohydride ions), surfactant, and molecular oxygen.65 Adsorption of nucleophiles to a nanoparticle surface increases the Fermi level of the silver nanoparticles, while withdrawal of electron density lowers the Fermi level.62,66 Hence, as illustrated in Figure 12, the presence of ionic species that (65) Pal, T.; Sau, T.; Jana, N. R. Langmuir 1997, 13, 1481.

This report describes a facile method for the preparation of an aqueous dispersion of 3.6 nm gold nanoparticles, electrostatically stabilized by weakly physisorbed DMAP ligands. The interaction of these DMAP ligands with the surface of the gold nanoparticles has been characterized. It was found by 1H NMR and IR studies that DMAP is adsorbed normal to the surface of the gold nanoparticle and interacts through the pyridine nitrogen. Adsorption is associated with a net transfer of charge from the DMAP molecule to the gold nanoparticle, leading to a stable dispersion with a net positive charge at the radius of shear. The nature of the DMAP-gold interaction is sufficiently weak to allow for their phase transfer to a dodecanethiol-containing chloroformic phase, generating monodisperse 4 nm DDTstabilized gold nanoparticles. The thermally promoted ripening of the nanoparticle dispersion 2 under at moderately elevated temperatures yields a dispersion of 11.3 nm gold nanoparticles. It has been shown that the presence of the unreduced chloroauric acid-DMAP complex is necessary to thermally ripen the DMAP-stabilized nanoparticles under these conditions. A mechanism for nanoparticle ripening is proposed, whereby the chloroauric acid-DMAP complex increases the surface energy and therefore the solubility of the nanoparticles, an effect that lowers the energy barrier for nanoparticle dissolution at a given temperature. Many nanoparticle synthetic methods involve reduction of a metal-ligand precursor, formed by the complexation of a surfactant with hydrogen tetrachloroaurate, to form the nanoparticles. It is envisaged that the procedure described in this report, whereby the addition of the unreduced metal-ligand precursor to a nanoparticle dispersion facilitates further nanoparticle growth at elevated temperatures, may be adapted to other nanoparticle systems, facilitating size control of nanoparticles of varying composition. It also offers the possibility to grow mixed metal nanoparticles by the addition of a different metal-ligand precursor (e.g., silver) to a nanoparticle dispersion of a given metal (e.g., gold). The possibility of adapting this process of nanoparticle growth to different nanoparticle systems is currently under investigation. Supporting Information Available: Additional spectra and micrographs as mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org. LA061261A (66) Mulvaney, P. Langmuir 1996, 12, 788.